Sodium vapor arc discharge lamps for use in street lighting, for example, are well known. Because the sodium ionizes at relatively high temperature, the lamps usually include a gas such as neon or xenon which ionizes readily and forms an initial arc. This arc is used to initially ionize mercury vapor, and the arc from the mercury vapor in turn ionizes the sodium. The start-up process for a cold lamp is typically in the order of five minutes.
The high voltage potential necessary to initiate an arc in the low pressure gases of the sodium lamp is derived from a ballast transformer connected to a constant current power source. In a typical installation, the ballast transformers for a number of lamps are connected in series across a moving coil type constant current power transformer. Because the transformer secondary of the ballast transformers looks at a very high impedance load until the arc is struck in the lamp, the voltage across the primary and secondary of the ballast transformer is initially quite high. With the rise in voltage with constant current, the power input to each lamp is therefore at a peak during initial start, and the ballast transformer will saturate. Th constant current transformer must therefore have a volt-ampere rating that is approximately four times the rating required to operate the lamps after they reach steady state operating condition. Since it is desirable to operate as many lamps as possible off of one power transformer, the relatively large start-up power requirement becomes a limiting factor.
In order to increase the number of lamps operated in a string from a single constant current transformer, one solution has been to time the starting of the lamps so that the starting is staggered. One such arrangement is described, for example, in copending application Ser. No. 091,539 filed Nov. 5, 1979, by the same inventor as the present application. However, the timing approach to limiting start-up loads by staggering the starting of the lamps requires that the ballast be tailored to provide optimum delay times, which must be selected by the contractor at the time of installation. Timing circuits further add to the cost of the equipment as well as resulting in additional maintenance costs.